Tuesday, October 13, 2009

When we left the introduction we were about to learn why population declines in neotropical migrants are occurring. The first focus is on challenges on the wintering grounds. My comments follow.

The Citizen's Guide to Migratory Bird ConservationObstacles in Latin America

Throughout much of Latin America, tropical forests are rapidly being converted to cropland and open grazing land by slash-and-burn agricultural techniques. Such forest destruction obviously creates problems for migrant birds that depend on forests for winter habitat. Another problem is geographical. The land mass inhabited by migrants in winter is much smaller than the vast breeding area -- all of North America. this means that wintering areas are often packed with five to eight times as many birds as are found in the same area on the breeding grounds. Therefore, the destruction of just a small amount of tropical forest can have a huge effect on some bird populations. Species restricted to a small wintering range, such as the Cerulean Warbler, are at the greatest risk. And areas with the greatest concentrations of migrants - Mexico, Central America, the Greater Antilles, and portions of the Andean mountain range in South America - also have some of the highest rates of deforestation.

Neotropical migrants wintering near cropland area are also threatened by pesticides, because the toxins concentrate in the birds' fat reserves. Some pesticides, including chlorinated hydrocarbons such as DDT, have been outlawed for use in the United States but are still used legally in Latin American and Caribbean countries. U.S. companies supply these pesticides.

My comments:As the first two entries this section has been excerpted in its entirety; the author information and article text current as of 1995.

When we hear about the effects of deforestation we almost always, if not completely always, consider the straight-up loss of habitat: less habitat means less birds. The ideal, of course, is to keep the habitat contiguous; if it must be degraded, keep the patches as large as possible, and joined to allow movement of individuals.

But what I find often escapes the general public's notice is that equal patch sizes are not equal in bird density. George Orwell might have said all acres of suitable habitat are created equal, but some of those acres are more equal than others. As the author's point out here, an acre of habitat in Latin America is more equal than an acre of habitat in North America. By highlighting the density difference in geography, that removing a hectare in the neotropics has a much more far-reaching effect than cutting a patch the similar size on the more-spread-out breeding ranges, they drive home an important aspect that reaches beyond mere habitat loss - it's a loss for far more birds than elsewhere in the Americas (this is not making the case that it's OK to cut larger swaths of the boreal forest).

Pesitcide-wise, I'm not sure where we stand these days. Are Latin American countries still using chemicals outlawed in the U.S.? I find myself wondering what improvements have been made in land use in Latin America, and what has deteriorated over the past 15 years.

Similarly, what about including recent results from studies on patch sizes, using familiar birds as specific examples? What's the critical size where you lose certain at-risk and common species? Or if clutch sizes or breeding success varies with the number of individuals in a patch?

Or do we start to lose the forest and only see trees wit specific details like these?

About My Blog

I write about nature and the outdoors, mostly relating to our local patch of the world in the Southern Tier of New York, but also our periodic travels to elsewhere. As a "mad-keen birder" I mostly focus on birds, as a father I often focus on nature seen through the eyes of a child. Anything that arouses my curiosity is fair game.

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